How to load a video in a MovieClip node
you are using incorrect key , the key is the name of the movie clip 

#load new movie clip
bpy.data.movieclips.load(clip_path)
#get the new movie clip
movie_clip = bpy.data.movieclips.get(clip_name)
#assign movie clip to the node
bpy.context.scene.node_tree.nodes['Movie Clip'].clip = movie_clip